This invention concerns an opto-mechanical system for detecting the height or position of a light reflective surface of an object. The system is useful in a variety of applications, including automatic focusing systems for probe-forming charged particle devices such as electron microscopes.
While the present invention is not limited to automatic focusing systems for electron microscopes, by way of example the present discussion will be in that context.
Manual focusing and stigmating of an electron microscope is time consuming, requires extensive training, and may not be consistent.
Automatic focusing improves througout, replaces the operator, can be more consistent (through improved precision), and requires no training. In the case of CD SEMs, throughput with operator focus is limited, but with automatic focusing according to this invention it can be significantly greater.
Automatic focusing approaches for electron beam microscopes may be classified into systems which:
1. Change the focus of the system objective or final lens in response to detected changes in the object surface height or position. The best focus is determined by processing the video signal.
The disadvantages of this approach are that it requires a good sample structure and a good video signal. It also usually requires re-stigmating after lens adjustment. Magnification will change and magnification errors may be introduced as a result of magnetic hysteresis.
2. Change the beam voltage instead of the final lens focus. The disadvantages of this latter approach are that it requires a good sample structure and a good video signal. It usually also requires re-stigmating. Magnification will change, and significant kV change may occur. If the sample height changes by 500 microns, kV will change from 1000 volts by as much 100 volts at a 5 mm focal length.
3. Move the sample up or down to a preset, stable, focus of the electron optics. The advantages of this approach are that there will be no change in magnification, beam voltage or stigmation. There will be no magnification error due to magnetic hysteresis. Focusing is similar to that done in a light microscope.
A number of techniques have been pursued in the past for deriving information indicative of the location of the examined object surface.
As described by Erasmus and Smith in their article, "An Automatic Focusing and Astigmatism Correction System for the SEM and CTEM", Journal of Microscopy, Volume 127, Part 2, August 1982, pages 185-189, a number of approaches have been attempted for automatic focusing of a scanning electron microscope (SEM). These are said to fall into one of two categories: those that determine the point of best focus from the derivative or gradient of the video signal, and those that analyze the power spectrum or diffraction pattern of the signal.
Erasmus and Smith also describe previous automatic focusing sytems for the CTEM (conventional transmission electron microscope). The earliest description of a focusing aid for a CTEM was the "wobbler" in which the parallax caused by tilting the illuminating beam produced an image shift which could be detected by the operator.
Attempts have been made to convert the CTEM image int an electrical signal and then to determine the defocus of the image from the power spectrum of the signal. Still other approaches involving electronic processing of a derived electrical signal are described by Erasmus and Smith.
U.S. Pat. No. 3,937,959 discloses a method for automatically focusing the electron beam of a scanning electron microscope which method involves adjusting the focal length of the electron beam condenser lens system as the beam is scanned and monitoring the output of detecting means for various scans to determine a minimum beam diameter.
British Patent No. 2,217,158 concerns automatic focusing apparatus for an electron microscope which monitors an integrated video signal for various values of objective lens current as that current is ramped through its range. Problems are described which result from lag and hysteresis. A solution is presented wherein the lens current is ramped up and down between its limits, integrated maxima are detected, and the lens current ramped up and down using these maxima as the new limits. The process is repeated until the range is sufficiently reduced such that the currents producing the maxima for the last two passes may be averaged to yield the correct focusing value.
Fushima et al, U.S. Pat. No. 4,990,776, alludes to such prior art systems. "Since it is necessary to irradiate the specimen with the electron beam several times until the in-focus position is attained, the specimen is subjected to damage and charging by the electron beam, thereby making it impossible to correctly detect a pattern when a semiconductor device or the like is to be observed, measured, and/or checked", (column 1, line 58).
Fushima et al describes a system which avoids use of the electron beam itself in the autofocusing process. In the Fushima et al system an optical beam is developed and focused on the object at the location of the electron beam focus. The electron beam system is automatically focused without the need to irradiate the object with the electron beam.
Fushima et al, however, suffers from a number of shortcomings associated with the way in which the optical beam and electron beam systems are integrated. The optical beam is folded onto the electron beam axis. By the use of a Schmidt-type imaging system, the optical beam is focused to a point coincident with the electron beam focus
The electron beam must be passed through an opening in the convex reflector of the optical imaging system. The result is a system in which neither the optical system nor the electron beam system can be optimized for maximum performance. The working distance of the device is fixed or extremely restricted. The Fushima et al system requires projection of an optical pattern on the object through the electron beam system. Further, the Fushima et al system is of questionable use in applications where it is desired to tilt the object, or wherein the object has a highly irregular surface topography.
An automatic electron beam focusing system is described in an article by Guillermo Toro-Lira entitled "Techniques for High Speed SEM Wafer Inspection for Production Applications", SPIE, Volume 1087, Integrated Circuit Metrology, Inspection and Process Control III (1989), pages 17-29. FIGS. 9 and 10 of the Toro-Lira system are reproduced as FIGS. 1 and 2 hereof. In the Toro-Lira system, a laser beam is focused on a wafer at the point of electron beam impingement (electron beam not shown). The reflected beam is collected by lens L2, reflected off a flat mirror M and reimaged onto the first focus by lens L2. See FIG. 1.
A beam splitter P directs the reflected beam into a lens L3 which images the second focus onto a detector D. As shown in FIG. 2, a signal derived from the detector is processed through "detector electronics", "control electronics" and "piezo translator power supply". The output from the power supply is supplied to a piezoelectric Z-translator which adjusts the position of the wafer in accordance with changes in the height of the wafer surface as detected by the optical system so as to maintain the wafer surface at a fixed location.
The Toro-Lira system has a fixed operating distance, with no provision for making gross adjustments in the operating distance. However, a truly useful automatic focusing system for an electron microscope must be able to accommodate a wide range of operating distances--that is, the distance between the objective or final lens of the electron optical system, and the surface of the specimen or object. The system must be able to accommodate objects of various thicknesses and topographical characteristics, electron optical systems having various focal lengths, and various arrangements of XYZ stages.
Secondly, a system for use in semiconductor wafer inspection, for example, must be capable of accommodating wafer tilts as great as 60 degrees. By the nature of the optical system of the Toro-Lira device, tilt of the wafer greater than a few degrees could not be tolerated.
It is believed that the Toro-Lira system will not readily accommodate an object surface having a "potato-chip" or otherwise wavy or irregular surface, as the reflected laser beam would not be collected with satisfactory efficiency.